1. Field of the Invention
The present invention relates to an overheating detection circuit for use with a power integrated circuit (IC) and, more specifically to an overheating detection circuit comprising a simple circuit construction which may be formed in the same substrate as that of a power IC, and which can produce a large output signal in response to a detection of a temperature within a range of detection temperatures.
2. Description of the Related Art
Because power devices are used under high voltage and large currents, an abrupt increase in a load connected to the device or a shortcircuiting of the load can result in large currents in excess of the rated current of the device flowing through the device, thereby creating a danger that the device will become excessively heated and, in an extreme case, that the device will be destroyed. To protect a power device against such thermal destruction, the temperature of the power device is constantly monitored and, when the temperature of the device exceeds a predetermined temperature or an overheating of the device is detected, some protection action is taken, for example, the power device is turned-off.
In monitoring a power IC, it is preferable to fabricate an overheating detection circuit including a temperature sensor into a substrate in which the power IC is formed in order to improve temperature sensitivity and to simplify the circuit arrangement. Typically, however, discrete elements have been used for forming a circuit which stops an operation of a power device upon detection of eddy currents or an overheating of the power device. In connection with this, a thermal sensor using a bipolar transistor as a thermo-sensitive sensor is described in E. Habekotte, Bull. ASE/UCS 76 (1985) 5, 9mars, pp. 272-276.
FIG. 10 illustrates a circuit arrangement of an overheating detection circuit of the prior art which uses the thermal sensor described in the above-cited reference. As shown in FIG. 10, a bipolar transistor 91, operating as a thermal sensor, is inserted into a feedback loop of an operational amplifier 92. An external constant current source (not shown) supplies a collector current I.sub.C to the transistor 91. The operational amplifier 92 produces an output voltage V.sub.1, which is equal in amplitude but of reverse polarity to a base-emitter voltage V.sub.BE of the transistor 91 As shown in FIG. 11, the base-emitter voltage V.sub.BE varies linearly with and is inversely proportional to temperature T. By properly amplifying the output voltage V.sub.1 of the operational amplifier 92 by another operational amplifier 94, an output voltage V.sub.out which varies linearly with respect to temperature T can be obtained.
A feature of the above thermal sensor is that the output voltage V.sub.out varies linearly with respect to changes in temperature, and that little error exists over a broad range of temperature change. However, when the thermal sensor is assembled, for example, into a power IC as an overheating detection circuit, one will encounter various problems. Because the circuit of FIG. 10 uses a constant voltage circuit with less temperature dependency and a comparator for comparison with the output voltage V.sub.out, a large scale of circuitry is required for the thermal sensor. Further, it is necessary to minimize the temperature dependency of the operational amplifiers 92 and 94, as well as a reference voltage source V.sub.ref. Otherwise, an error arising from the large temperature dependency of each of these components will greatly and adversely influence a detected temperature. To remove the adverse influence, most of the circuitry except the bipolar transistor 91 is fabricated and contained in a separate package which is placed at a location such that it is not influenced by the temperature of the power IC. Accordingly, when fabricating the circuit into a power IC package, it is necessary to solve the problems associated with the substrate temperature and the increased size of the circuit.
The conventional thermal sensor provides an output signal which varies linearly over a broad range of temperature changes. To the contrary, the conventional overheating detection circuit is designed such that when the temperature of a power device reaches approximately 150.degree.-180.degree. C., it determines that the device temperature has reached an overheating temperature, and produces an output signal. Therefore, the overheating detection circuit must produce an output signal which must vary greatly in accordance with a relatively small temperature range. Thus, the performance requirements of the conventional thermal sensor and overheating detection circuit are quite different from each other.